The present invention relates to control of direct current (DC) electric motors and, more particularly, to an open-loop speed regulator circuit for a series DC motor.
Performance requirements for electrically powered, low voltage drive systems, particularly the torque-speed requirements, generally dictate the use of DC motors. Although the DC motor may be any of the well-known types such as, for example, series wound, compound wound or separately excited, the series wound motor possesses certain characteristics uniquely suited for use in low voltage battery powered systems. Conventionally, the motor is energized by a chopper circuit which incorporates a controllable switch in series between the motor and a DC power source. By cyclically opening and closing the switch while varying the time ratio of the open time to the closed time of the switch, the magnitude of power supplied to the motor can be regulated. Such chopper circuits are well known in the art.
As will be appreciated, the act of cyclically opening and closing the switch in a chopper circuit serves to periodically connect the motor essentially directly to the DC source, although some inductance is generally in the motor current path. The average voltage on the motor is proportional to the time ratio or duty factor of the chopper circuit.
Although the motor voltage is determined by the chopper duty cycle, the speed of the motor rotor is determined not only by the motor voltage but also by the load applied to the motor. Thus, in order to regulate motor speed, it is necessary to vary the chopper duty cycle as a function of motor load. The prior art has typically utilized a tachometer to monitor motor speed and to provide a control signal to adjust chopper duty cycle for regulating motor speed. However, tachometer circuits are relatively expensive to implement.